Change to sql database does not get save

Question

hello friend I have downloaded a database from code plex site to my local machine in sql 2008 & I change the index of one attribute & tries to save that database but i got one error message box "error wear encountered during save process some database object ware not saved unable to modify table ,Cannot create Xml index without a clustered primary key.please help & i am not creating any index on xml actually i have removed an index which was there in original table & goes to save the table I got this error i am not creating any index